Currently, display techniques developed for new screen types are optimised to reduce or eliminate flicker. The “100 Hz” concept or doubling of the scanning frequency first appeared on cathode ray tubes then liquid crystal monitors or screens became the reference for computer screens because of the almost complete absence of flicker due to their support type addressing mode. Current plasma screens with addressing by temporal modulation and picture repetition have, for the human eye, a behaviour close to that of 100 Hz cathode ray tube screens. All of these display techniques have enabled reduction of flicker to the detriment of the display of animated scenes. Of course there are motion compensation techniques but these are rarely used in television screens and their precision is not always sufficient to have an appreciable impact on displayed pictures. Moreover, for LCD screens, a reduction of their response time is too often presumed to be the solution to improve the quality of animated pictures and yet, even with a null response time, the LCD screen continues to produce a blur effect on objects in motion due to their support type addressing mode. Indeed multiple contours can also appear when the refresh frequency is increased, for example a double contour appears on objects when the screen refreshing frequency is 100 Hz. All of these effects of flicker, of blurring and of multiple contours are described in more detail in the following paragraphs.
The flicker effect and more particularly the “large area flicker” effect is linked to the refresh frequency and/or the screen addressing mode. The limit of perception of large area flicker by the human eye is approximately 60 Hz. If the refresh frequency is greater than this limit, the flicker effect is not or is hardly perceived by the human eye whatever the addressing type. Likewise, when there is support type addressing (as for LCDs), the flicker effect is not perceived. Therefore standard LCD screens (50 or 60 Hz addressing) do not introduce a flicker effect but do introduce a blur effect when the pictures comprise movements. In pulse type screens (such as cathode ray tube screens and plasma screens where the light is concentrated mainly on a reduced portion of the frame period) the flicker effect exists only if the refresh frequency is less than 60 Hz. Doubling of the refresh frequency (100 Hz or 120 Hz) eliminates this effect but introduces multiple contours on objects in motion in the pictures as illustrated further on.
The blur effect generally appears on the motion transitions in the picture. FIG. 1 illustrates this effect on a transition between a grey area and a black area in a picture displayed by an LCD screen (support type addressing). The left part of FIG. 1 illustrates the case where the transition is static on one or more successive video frames and the right part illustrates the case where the transition moves towards the right. In these two parts of the picture, the horizontal axis represents space and the vertical axis represents time. As can be seen on the left part of the figure, in the absence of motion, there is no blurring and the transition perceived by the eye is clear. In the right part of the figure, in the presence of motion, the eye follows the motion and integrates the light in the direction of the motion. A blurring effect then appears on the transition.
Finally the “multiple contours” effect has the same causes as the blurring effect. However, this only appears on fine objects in motion such as text. As previously indicated, this effect appears when the refresh frequency is multiplied by n, n being greater than or equal to 2. FIG. 2 illustrates this effect for a picture displaying the word “Thomson” in grey on a black background. The refresh frequency of the screen displaying this text is doubled. The left part of FIG. 2 illustrates the case where the text is static on several successive video frames and the right part illustrates the case where the text moves towards the right. In these two parts of the picture, the horizontal axis represents space and the vertical axis represents time. As shown on the left part of the figure, in the absence of motion, there are no double contours. As shown on the right part of the figure, in the presence of motion, the eye follows the motion and integrates the light in the direction of the motion. A double contours effect appears on the word “Thomson”.
To reduce these effects of blurring and of multiple contours, the use of motion compensation is known. This technique consists in modifying the video content, for example for one 100 Hz video picture in two, according to the motion detected. This technique is illustrated by FIG. 3 for a pulse type screen. FIG. 3 shows a transition between a grey area and a black area in a picture. The left part of FIG. 3 illustrates the case where the transition moves towards the right without motion compensation and the right part illustrates the case where the transition moves towards the right with motion compensation carried out in one 100 Hz picture in two. In the two parts of the figure, the horizontal axis represents space and the vertical axis represents time. As the left part of the figure shows, in the absence of compensation, there is blurring at the level of the transition perceived by the eye. Likewise, a double contour effect appears when text is displayed. As the right part of the figure shows, in the presence of compensation, the blurring effect disappears. The same is true for the double contours.